In all patients,the patients' two lungs will be mechanically ventilated with intermittent positive pressure ventilation using fraction of inspired oxygen (FiO2) of 0.5 in air, tidal volume (VT) of 8 mL/kg (predicted body weight), inspiratory to expiratory [I: E] ratio of 1:2.5, a positive end-expiratory pressure (PEEP) of 5 cm H2O, respiratory rate (R.R) will be adjusted to achieve an arterial carbon dioxide tension (PaCO2) 35-45 mm Hg, peak inspiratory pressures will be limited to 35 cm H2O and a low flow of fresh gas (<2 L/min) in a semi closed circuit system.